1. Field of the Invention
The present invention relates to a charge transfer device and, more particularly, to an improved electrode construction for use in a charge transfer device.
2. Description of the Prior Art
Charge transfer device (CTD) represented by a charge coupled device (CCD) or a bucket-brigate device (BBD) are applied to a number of electrical devices such as memories, image pick-up devices or delay lines, due to their functions of charge storage and charge transfer. One of the important characteristics of the CTD used in the electrical devices is a charge transfer efficiency. This is because a charge transfer through a large number of transfer gates, for example several hundreds is required in recent applications. As a result, a tiny loss in a single transfer gate will produce a very large loss upon the final output. Therefore, there is a strong demand to improve the transfer efficiency of CTDs.
This transfer efficiency strongly depends not only on the waveform of the clock pulses, but also on both the strength of the electric field accelerating the transfer of the charges and the length of the respective gates. Especially, the influence of the accelerating electric field becomes dominant when the frequency of the clock pulses is high, in case of the same length of the gates.
A charge transfer device according to the prior art has a number of transfer electrodes formed on but isolated from a channel region which is a region of a semiconductor substrate for transferring charges. These transfer electrodes are arranged so as to intersect that channel at right angles. Stepped potentials are established in the channel region by clock pulses applied to these transfer electrodes so that charges are transferred by the potential differences between regions under the adjacent transfer electrodes. Those stepped potentials are not uniform under the respective transfer electrodes but is more or less inclined. This inclined potential distribution generates fringing fields which accelerate the charge transfer. As the fringing fields are made more intense by, for example, shortening the length of the transfer electrodes in the charge transfer directions, the charge transfer can be effected more completely. However, this shortening causes another drawback of a small transfer capacity of charges. In case of same lengths of the transfer electrodes, so long as the transfer electrodes have a band shape that intersects the transfer direction of the channel region at right angles, however, it is impossible to expect any significant intensification of the fringing fields. This means that any significant improvement of the charge transfer efficiency cannot be expected.